Golf Clubs. The game of golf is played with three basic club types: putter, iron, and wood. Each of these clubs is formed of a head which strikes the ball and a shaft attached to the head and which is gripped by the golfer to control the head motion. The club head is mounted to the shaft by inserting the shaft into a receptacle provided on the head (typically referred to as a "hosel"). The putter head has a flat, generally vertically oriented surface to strike the ball and cause it to roll on the surface of the ground. The iron has a flat striking surface that is oriented at an angle inclined from the vertical to cause the ball to travel at varying angles upward depending on the club. Woods have a generally flat and inclined striking surface on a bulbous body, which is intended to reduce aerodynamic drag during the swing. The reduced drag allows higher club head velocity for increased distance. The rules of golf are provided by the United States Golf Association and the Royal and Ancient Golf Club of St. Andrews. These rules do not allow moving parts, appendages, holes through the club head, or club heads that are not plain in shape.
Each type of club head has a "sweet spot" or center-of-percussion which is the location on the striking surface at which the center of mass of the club head will be aligned directly behind the center of mass of the ball during impact. When a golfer hits a ball with the sweet spot of the club head, the minimum amount of energy is transmitted to the golf club from the ball and the resulting distance the ball travels is maximized. When the sweet spot is not struck, the misalignment of the centers of mass results in a moment that tends to twist the club head. This twisting serves to transmit energy to the golfer that could have been imparted to the ball. The twisting also results in some divergence of the ball from its intended path due to the angle of twist and the resulting spin imparted to the ball.
Putters. The striking surface of a putter is typically aligned within one degree of vertical, as its primary function is to cause the ball to roll smoothly on a relatively flat surface. A putter head is a rigid structure with the hosel placed at any location on the head. Sufficient rigidity of the putter head is simple to achieve, as the impact velocities are low. Some efforts to improve the "feel" of putters have gone towards use of different materials such as brass or copper. Other efforts to improve the feel have involved modifications of the striking surface by providing an insert of resilient material. The only other substantial design modification for putters has been limited efforts to improve the moment of inertia about a vertical (or "yaw") axis. These efforts have included the redistribution of mass to the inner and outer lateral ends of the striking surface relative to the direction of travel (or "heel" and "toe"). In addition, putter designers have created the "mallet" putter that accomplishes mass redistribution by extending the putter head in a semicircular fashion to the rear of the striking surface.
Irons. The inclination from the vertical of the striking surface of an iron golf club head is commonly referred to as its "loft" and is measured in degrees. Irons are commonly available as a driver or #1 iron through a #9 iron and further as wedges for even shorter distances and sand shots. The #1 through #9 irons typically have from 15 to 45 degrees of loft while the wedges have from 45 to 65 degrees of loft. As the loft decreases, the shaft length increases to provide a higher club head velocity. A typical #9 iron or wedge has an approximately 36" shaft, whereas a #1 iron has a 40" or longer shaft. The mass of each head is usually matched to the shaft length to provide a constant centrifugal force or "swing weight."
The iron-type head is typically a rigid structure as there is sufficient mass available to design for high rigidity. Recent design improvements and use of high-strength materials have allowed redistribution of mass to increase the moments of inertia of the head. These modifications have resulted in irons with "perimeter weighting" and "oversized" irons. Perimeter weighting is redistribution of the mass to the perimeter of the striking surface to increase the moments of inertia. Oversized irons have an increased size of the striking surface through design and the use of high strength, low-density materials. This increase in size is accomplished specifically to increase the distance of the mass of the club head from its center of gravity--again increasing the moments of inertia.
Woods. A wood generally has less loft and a longer shaft than an iron in order to achieve greater distances. Woods are commonly available as a driver or #1 wood through a #9 wood with lofts ranging between 5 and 30 degrees and shaft lengths ranging between 48" and 41" respectively. Like the irons, the combination of smaller loft angle and longer shaft length increases the club head speed and resulting distance for the driver.
The original (and still available) construction of a wood-type head was to form a club head constructed of persimmon, a wood with low density and high stiffness (or modulus of elasticity). These club heads are made of solid wood resulting in a rigid body. As a result, the natural wood head transfers maximum energy to a ball struck at the sweet spot. The design was eventually modified by the application of metal to some portion of the striking surface and the bottom surface (or sole) for increased durability. The body volume provides a mass distribution with greater moments of inertia about the point of contact with the ball than a comparable iron of its time and also serves to significantly reduce aerodynamic drag. A solid wood club head has the disadvantage that its density limits its size and the resulting inertial properties, so the resulting size of the sweet spot is relatively small. The shaft length and club head mass are designed to generate a "swing weight" in a range which allows the golfer to achieve high circumferential velocity of the club head while maintaining proper control of its path.
Recent applications of materials and design features have revolutionized the design of wood-type heads. This has resulted in wood-type heads made of metal (commonly known as metal woods) and in wood-type heads made of polymer composite materials. The first application was the use of steel to replace the persimmon wood. It is likely that the main advantages sought were reduced manufacturing cost and increased durability. This application of material resulted in a hollow body to maintain the proper mass. A possibly unexpected benefit was a significantly improved mass distribution--with the mass all moved to the surface of the club head, the moments of inertia were significantly increased. This advantage is similar to that obtained through perimeter weighting used primarily for irons, but is actually more effective at increasing the moments of inertia. The use of a hollow body also introduced a problem that has to be dealt with in all hollow, wood-type head designs. This is due to a decrease in rigidity of the head structure as a result of the hollow design. To maintain the weight of the head within acceptable bounds, the walls must be fairly thin resulting in increased structural flexibility. A number of patents during this century have proposed stiffening features to the hollow design in attempt to overcome this problem. A structure that flexes during impact will absorb greater energy and, therefore, transfer less energy to the golf ball.
The next evolution for wood-type heads was to take advantage of higher strength materials by increasing the size of the club head, resulting in what is known as the "oversized wood." Without further information, the layman could easily conclude that the size of the club head directly provides the advertised larger "sweet spot" by providing a larger striking surface. However, the advantage is actually achieved through the increased moments of inertia provided by the larger size. The first of the improved materials used was stainless steel, which has the advantage of being corrosion resistant. With stronger materials, the structural rigidity could be improved, the head could be made larger with similar weight and rigidity, or the head could be made lighter to allow a longer shaft with higher impact velocity. This evolution was followed by the use of titanium which is lighter than steel for the same strength. Many manufacturers have used titanium to provide club heads that are over twice as large (in volume) as the original wood heads. Titanium has approximately half the density of stainless steel, but also has only half the stiffness. In this case, the lighter weight allows for thicker walls, which provides improved rigidity for the same mass of material--resulting in a somewhat even trade.
During the same timeframe as the introduction of titanium, graphite fiber reinforced epoxies and similar composites have been used in golf club heads. This material has one-third the density of titanium and, as a result, can provide lighter weight and/or larger head size. It is likely that similar stiffness to that provided by titanium heads can be achieved with composites, but the overall advantages remain to be seen.
The next evolution of the wood-type head will likely be the use of even more advanced materials such as metal-matrix composites, ceramics, and ceramic-matrix composites. The use of these materials began with application to face inserts to provide a rigid striking surface. However, this still left body flexure as a source of energy absorption while striking a golf ball. In U.S. Pat. No. 3,975,023, Inamori provides an early example of the use of a ceramic faceplate. The increased application of ceramics is inevitable, as the recent progress in the high-technology industry has yielded ceramics with high strength, high rigidity, and reasonably high fracture toughness.
In U.S. Pat. No. 5,342,812, Niskanen et al. disclose the use of such advanced materials through a method patent. This patent describes a mass in the shape of a golf club head made of either a ceramic- or metal-matrix composite material with either a metal- or ceramic-matrix insert intended to be used as a striking surface. The practical application of Niskanen's claims is not entirely clear. The logic that has resulted in hollow wood-type heads and their resulting thin walls is not obviated by the application of advanced ceramic- and metal-matrix composites. The achievable density is in the realm of 30% less than that of titanium. The patent makes vague references to tailoring material properties, but it would be difficult to cast or press a solid wood-type head (as implied by the patent) which would have the desired size and still be light enough to be useful.
The replacement of a hollow titanium shell with a hollow ceramic- or metal-matrix composite shell would allow somewhat thicker walls and resulting greater stiffness. However, ceramic-matrix composites have less than 20% of the fracture toughness and their durability would be in question even with the increased wall thicknesses obtainable. Niskanen did not describe such an application of these new materials. Manufacturing a good quality sample in the desired shape would likely be difficult and expensive at best. The use of a metal-matrix composite would allow higher fracture toughness, but the higher density of the materials would eliminate the weight advantage and corresponding wall thickness gains over titanium and the same difficulties would likely be encountered in manufacturing.